This invention relates to the determination of engine mass air-flow and more particularly to a method for deriving an indication of the mass of air available for combustion within a cylinder of a crankcase scavenged two-stroke engine.
In a crankcase scavenged two-stroke engine, each individual cylinder has its own separate crankcase chamber. During portions of the engine operating cycle, air is inducted into each crankcase chamber, compressed while the crankcase chamber is decreasing in volume, and then transferred to the associated cylinder combustion chamber, where it is mixed with fuel for ignition.
In order to effectively control the emission and performance characteristics of such an engine, it is necessary to know the mass of air available at the time of combustion within each cylinder. Once this information is known, the air-fuel ratio can be adjusted accordingly to achieved the desired emission and performance objectives.
Conventional hot wire or hot film sensors can be used to measure the total air-flow per cycle, in a two-stroke engine, however, these sensors tend to be relatively expensive, fragile, and easily contaminated by dirt in the air flow. Alternatives have been proposed for eliminating conventional mass air-flow sensors in crankcase scavenged, two-stroke engines, and for estimating the individual mass air-flow per cylinder. These techniques are described in U.S application Ser. No. 377,383, filed July 10, 1989 now U.S. Pat. No. 4,920,790, and U.S. application Ser. No. 409,377, filed Sept. 19, 1989, now U.S. Pat. No. 4,920,789 which are copending with the present application and assigned to the same assignee. In each case, the mass of air trapped within a crankcase chamber is determined as a function of pressure, volume, and temperature of the air during crankcase compression, prior to the transfer of air to a cylinder combustion chamber. Each technique requires correction factors to account for incomplete transference of air between crankcase and combustion chambers, and to account for leakage of the transferred air out of the combustion chamber prior to cylinder exhaust port closure.